1. Field of the Invention
The present invention relates to facsimile systems and more particularly to an improved synchronizing system that is relatively immune to timing errors that might be introduced by a "noisy" transmission link.
2. Related Prior Art
The present invention is intended to be an improvement of and modification to the Graphic Indicia Video Signal Acquisition Transmission and Reproduction System of U.S. Pat. No. 3,843,839, issued Oct. 22, 1974, to John Scott Campbell, et al., and assigned to the assignee of the present invention. A system for controlling brightness and contrast which is useful in the present system was disclosed in a patent to the applicant, U.S. Pat. No. 3,952,144, issued Apr. 20, 1976, and also assigned to the assignee of the present invention.
A system for speeding the data acquisition process, especially when dealing with documents that include non-informational areas, intended to function with the above-identified transmission and reproduction or "facsimile" system has been disclosed in a co-pending application of Kolker et al., Ser. No. 696,744, filed June 16, 1976, now U.S. Pat. No. 4,095,248, which has also been assigned to the assignee of the present invention.
Other prior art facsimile systems have been discussed in the above-identified references and the state of the art has been fairly well documented. It is the object of the present invention to provide an improvement to the systems heretofore disclosed, which tends to eliminate the problems caused by timing instability introduced by the transmission link between substantially identical transceivers.
In the prior systems, a transceiver functioning as a transmitter generated and transmitted an "End Of Frame" signal and a series of SYNC signals which modulated a carrier suitable for voice grade communication links such as telephone circuits. A substantially identical transceiver, functioning as a receiver utilized a synchronizing signal to operate a receiving device in substantial synchronism with the transmitter at the beginning of each line scan so that, except for the delay introduced by the communication link and which may be disregarded, the receiver "followed" the transmitter in the reproduction of the transmitted information.
Accordingly, the received End Of Frame signal reset and cleared all operating circuits in preparation for the start of a transmission and the received SYNC signal (which includes a pre-SYNC and post-SYNC portion) generated the necessary signals to increment the vertical deflection circuits and provide flyback and start-up of the horizontal sweep circuits. In order to maintain the transmitter and receiver in synchronism, the SYNC pulse which was transmitted was also utilized at the transmitter to accomplish the same functions for the transmitter that were accomplished at the receiver, such as control of duration of vertical step and timing of the beginning of the horizontal sweep.
The system is operable in a scan mode when transmitting information bearing segments of a document and in search modes when searching segments of the document to determine if they are information bearing or non-information bearing portions. The above-identified Kolker et al. application teaches a system which uses various modes to distinguish between information and non-information bearing portions of a document, and selects the scan speed and vertical step magnitude accordingly.
The SYNC signal in scan passes, search passes and combined scan-search passes continues to be produced and is transmitted at the end of each horizontal pass, to determine by its occurrence the start time or horizontal release time of the next pass and to control, by its duration, the vertical step magnitude between the present pass and the next pass. Since any pass can include either high and/or low speed portions, the duration of a given pass is not predictable and thus the time of arrival of the transmitted SYNC pulse cannot be anticipated. If phase instabilities, distortions, noise, etc. are introduced into the communication link resulting in timing errors, then any delay in recognizing and utilizing the transmitted SYNC signal would result in a line which may be displaced relative to its proper location. These errors, if occurring on successive lines, might result in blurred or illegible information, since vertical lines on the original copy would not necessarily be reproduced as strictly vertical lines.
Further, since a plurality of horizontal scans are required to build up a line of print, the individual characters may become distorted and possibly illegible. As noted, the duration of the SYNC signal is directly used to indicate the magnitude of the vertical increment that must be stepped to each next scan. Where the vertical distance between successive information areas is important, if the vertical registration of the copy does not correspond to that of the original, errors could result, especially if vertical dimensioning is important.
Since the transmission link between transceivers will generally be voice grade telephone lines, the effects of the path on the transmitted SYNC signal are significant. The path may introduce short noise pulses or transients whose onset can be misinterpreted as the initiation of the SYNC signals. The path may introduce delays or distortions which will tend to vary the duration time, thereby affecting the time occurrence of either or both the rising and falling edges of the SYNC signal. The cumulative effect on the SYNC pulse is to create uncertainties as to the exact time of initiation and termination as well as a "stretching" or "shrinking" of the actual duration time of the pulse. As a result, transmission link timing errors may occasionally result in poor quality documents being created by the receiver.
As may be noted from the Campbell et al. patent, the magnitude of vertical deflection or of the incremental vertical step between successive scans or passes is determined by the time duration of the SYNC pulse that is generated at the end of each pass to synchronize the start of a new pass. That capability was exploited in the co-pending application to Kolker et al., supra, and is also utilized in the present invention.
According to the present invention, a more precise synchronism is acheived between remote transceivers by the introduction into each transceiver of relatively high frequency, precision oscillators as internal references. When a transceiver is in its "sending" mode, its synchronization signal is under "control" of a precision oscillator. When a like transceiver is in its "receiving" mode, its synchronization is under the gross control of the transmitter and further under precise "control" of its own precision oscillator.
Each precision oscillator is counted down to provide a precisely timed clock which combines with various signals to produce SYNC signals and internal control signals that direct scanning circuits of the sending and receiving elements of the facsimile system.
In the preferred embodiment, the precisely timed clock signals are derived from a 5.21088 Mhz frequency oscillators and are placed into phase alignment at the beginning of a document transmission by a signal that is generated at the transmitter, thereby simultaneously resetting the frequency dividers in both the transmitter and the receiver to the same starting condition.
The combination of the low relative drift rate of the precision oscillators and the initial phase alignment maintain the transmitter and receiver in synchronism throughout the time required to transmit a document.